1. Field of the Invention
The present invention relates to an active type electroluminescence (EL) display device in which organic EL elements are driven using thin film transistors (TFT).
2. Description of the Related Art
Organic EL elements are suited for liquid crystal displays with reduced thickness because organic EL elements are self-emissive and therefore do not require a backlight. Furthermore, organic EL elements do not restrict the viewing angle of display devices in which they are employed. For these reasons, it is widely expected that organic EL displays will be as the primary display devices of the next generation.
Organic EL display devices are commonly classified as being either passive matrix type, having a simple matrix structure, or an active matrix type employing Thin Film Transistors (TFTs). In a conventional active matrix device, a drive circuit as shown in FIG. 1 is employed.
In FIG. 1, numeral 70 denotes an organic EL element. A drive circuit for one pixel comprises a switching TFT 71 which turns on and off according to a selection signal SCAN. In the TFT 71, a display signal DATA from a display signal line 75 is applied to the drain, while the selection signal SCAN from a selection signal line 76 is applied to the gate. The drive circuit also comprises a capacitor 72 connected between the source of the TFT 71 and a predetermined direct current voltage Vsc. When the TFT 71 is turned on, the capacitor 72 is charged with the display signal supplied from the display signal line 75. The capacitor 72 retains the charge voltage VG when the TFT 71 is turned off. The drive circuit further includes a driving TFT 74. In the TFT 74, the drain is connected to a power source line 77 that supplies a power source voltage Vdd, while the source is connected to the anode of the organic EL element 70. The retained voltage VG from the capacitor 72 is supplied to the gate of the TFT 74, which allows the TFT 74 to drive the organic EL element 70 by a current. The cathode of the organic EL element is typically connected to a ground (GND) potential. The power source voltage Vdd is a positive potential of, for example, 10V. The voltage Vsc may be the same potential as Vdd, or alternatively, a ground (GND) potential.
As shown in FIG. 2, the organic EL element 70 comprises an anode 51 constituted by a transparent electrode made of ITO (indium tin oxide) or a similar material, and a cathode 55 composed of a magnesium-indium alloy. Laminated between the anode 51 and the cathode 55 are, in order, a hole-transport layer 52 composed of MTDATA(4,4',4"-tris(3-methylphenylphenylamino)triphenylamine), an emissive layer 53 composed of TPD (N,N'-diphenyl-N, N'-di(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) and rubrene, and an electron transport layer 54 made of Alq3 (8-hydroxyquinoline aluminium). Light is emitted when a hole injected from the anode 51 and an electron injected from the cathode 55 recombine within the emissive layer 53. The light radiates outside through the side of the transparent anode 51, as indicated by an arrow in the figure.
The driving TFT 74 is configured by forming on a glass substrate 60, in order, a gate electrode 61; a gate insulating film 62; a poly-silicon thin film 65 including a drain region 63, a channel region, and a source region 64; an interlayer insulating film 66; and a planarization film 67. The drain region 63 is connected to a drain electrode 68 constituting the power source line 77 (see FIG. 1). The source region 64 is connected to the transparent electrode 51 serving as the anode of the organic EL element.
In a conventional arrangement, the cathode of the EL element is connected to the ground potential. The anode is connected to the TFT 74 for driving the EL element by current, and this TFT 74 is supplied with a fixed positive power source voltage Vdd. With such an arrangement, the maximum current flowing in one EL element is fixed, and the emissive luminance of the pixel is therefore also fixed.
When displaying an image in which light-emitting pixels dominate a large area in the overall display screen, if the luminance of the light-emitting pixels is too high, the displayed image may become glaring or bright, and unpleasant to the viewer's eyes. The above-mentioned power source voltage may therefore be lowered to set a lower maximum current value, such that the pixels emit light at a slightly reduced luminance. Under such a setting, the emissive luminance becomes similarly reduced when displaying an image in which light-emitting pixels cover only a small area of the overall display screen, producing a display image having a low contrast. However, if the power source voltage is set at a high level to allow the pixels to emit light at an increased luminance suitable for an image having a small area covered with light-emitting pixels, the display screen again becomes glaringly bright in the viewer's eyes when displaying an image having a large area dominated by light-emitting pixels. Furthermore, power consumption will be undesirably increased.